1. Field of the Invention
The present invention relates to the deposition of II-VI semiconductor films, and, more particularly, to the deposition of ternary chalcogenide semiconductor films, such as HgCdTe and HgZnTe, onto silicon substrates.
2. Description of Related Art
Ternary II-VI semiconductor films find use in many infra-red (IR) applications, such as in IR focal plane arrays (FPAs). Examples of such ternary II-VI semiconductor compounds include HgCdTe and HgZnTe, which are also known as chalcogenides.
The current size of HgCdTe focal plane arrays which are fabricated from homoepitaxial material is limited to 64.times.64 pixels because of delamination problems with the silicon readout to which the FPA is bonded by indium bump technology. The delamination problem is a consequence of the large thermal expansion mismatch between silicon and the CdZnTe substrate on which the active HgCdTe layers are grown.
Direct growth of HgCdTe on silicon not only accommodates this mismatch by forcing the FPA to expand and contract with the silicon substrate but allows HgCdTe growth on large area high quality substrates which are not available for homoepitaxy. Direct growth of HgCdTe on silicon is difficult, however, due to the large lattice mismatch (approximately 20%) and the polar versus nonpolar nature of HgCdTe versus silicon.
Buffer layers of ZnSe (4% lattice mismatch with silicon) and ZnTe (8% lattice mismatch with silicon) can De grown to step grade the transition to HgCdTe, but direct growth of these materials on silicon also yields poor quality or polycrystalline films.
Recently, it has been reported that high quality layers of ZnSe can be grown on silicon by molecular beam epitaxy (MBE) if the silicon substrate is terminated by a layer of arsenic atoms; see, e.g., R.D. Bringans et al, Materials Research Society Symposium Proceedings, Vol. 242, pp. 191-202 (1992). It is hypothesized that the function of arsenic is to passivate the silicon surface by fully coordinating the surface layer as well as providing an initial polarization of the surface for subsequent II-VI growth.
The first demonstration of the MBE growth of ZnTe on As-terminated silicon was recently the subject of patent application Serial No. 08/043,644, filed Apr. 6, 1993, and assigned to the same assignee as the present application.
Successful growth of ZnSe on Si terminated with a monolayer of As (henceforth referred to as ZnSe//As:Si) achieved by MBE, however, does not mean that the same approach will be successful in a metalorganic MBE (MOMBE) environment. Due to the large background pressure of organic radicals in the MOMBE environment generated by the thermal pre-cracking of the metalorganic sources, it has been believed that MOMBE growth on silicon would not be possible. It is well-documented in the surface science literature that hydrocarbon radicals, in particular methyl radicals, irreversibly adsorb on silicon surfaces and decompose to form carbides on the surface when heated; see, e.g., M.L. Colaianni et al, Chemical Physics Letters, Vol. 191, pp. 561-568 (Apr. 17, 1992). Studies have also shown that organoarsenic sources will strongly interact with surface oxides to leave carbonaceous contamination on the growth surface; see, e.g., P. Kaul et al, Journal of Crystal Growth, Vol. 123, pp. 411-422 (1992).
Simply adding a solid arsenic source to the MOMBE growth chamber is not the best solution, as the compatibility of solid sources in a metalorganic environment is not established. Accordingly, a method is needed for the formation of a monolayer of arsenic on silicon substrates using MOMBE.
Similar considerations also apply for the deposition of arsenic on silicon substrates using other metalorganic deposition procedures. An example of such other deposition procedure is MOVPE (metalorganic vapor phase epitaxy), also known as MOCVD (metalorganic chemical vapor deposition), which is carried out under vacuum conditions not as high as those employed in MOMBE. For example, the base pressure after bakeout for MOMBE will be ultrahigh vacuum (10.sup.-10 to 10.sup.-11 Torr) MOVPE chambers are not baked and have base pressures more typically on the order of 10.sup.-4 to 10.sup.-7 Torr, depending on how they are pumped.
Thus, a need remains for the epitaxial growth of good quality binary and ternary II-VI semiconductor films on a silicon substrate.